The present invention relates to high resolution medical monitoring apparatus. The invention is particularly useful in electrocardiograph (ECG) apparatus for monitoring ECG signals accompanying heart activity, and the invention is therefore described below particularly with respect to this application.
In ECG apparatus, a plurality of electrodes placed on the patient detect electrical signals accompanying heart activity and feed these signals through a multi-conductor cable to the ECG recorder. The ECG apparatus is usually used in environments where electrical noise from adjacent power lines is inductively coupled to the ECG cable, thereby adding a significant amount of electrical noise induced from the electrical supply lines to the ECG signals which are relatively weak. Accordingly, the S/N (signal-to-noise) ratio of the ECG signal can be fairly low. Moreover, the 50 Hz or 60 Hz frequency of the electrical supply line noise is within the frequency spectrum of the ECG signals.
Various types of filters have been devised for increasing the S/N ratio in medical monitoring equipment in general, and in ECG apparatus in particular, but efforts are continuously being made to devise new filter networks to increase the S/N ratio because of the importance of the ECG signal in providing information on the condition and performance of the heart.
According to one aspect of the present invention, there is provided medical monitoring apparatus to be used in an environment of electrical supply lines of a particular line frequency, for monitoring a particular body activity and for producing an electrical output signal corresponding to the monitored body activity; the apparatus, comprising: a first filter network connected to receive the electrical output signal and tuned to the supply line frequency for rejecting the line frequency component of the electrical output signal; a second filter network connected to receive the output of the first filter network and also tuned to the line frequency for rejecting the line frequency component of the electrical output signal; and a high impedance coupling between the first and second filter networks.
According to further features in the preferred embodiments of the invention described below, at least one of the filter networks, and preferably both, include a first capacitor and a second capacitor connected in series with each other to ground; and an operational amplifier having one input connected from a point between the second capacitor and ground, an output connected to a point between the first and second capacitors, and a second input connect to the output of the operational amplifier.
In one described preferred embodiment, the high impedance coupling between the first and second filter networks is an electrical resistor network; and in a second described preferred embodiment, the high impedance coupling between the first and second filter networks is a buffer amplifier.
According to another aspect of the present invention, there is provided medical monitoring apparatus to be used in an environment of electrical supply lines of a particular line frequency, for monitoring a particular body activity and for producing an electrical output signal corresponding to the monitored body activity; the apparatus, including a filter network connected to receive the electrical output signal and tuned to the supply line frequency for rejecting the line frequency component of the electrical output signal, the filter network comprising a first capacitor and a second capacitor connected in series with each other to ground; and an operational amplifier having one input connected from a point between the second capacitor and ground, an output connected to a point between the first and second capacitors, and a second input connect to the output of the operational amplifier.
As will be described more particularly below, particularly good results are obtainable when the medical monitoring apparatus monitors electrocardiograph (ECG) signals accompanying heart activity.
Further features and advantages of the invention will be apparent from the description below.